501 Not Implemented
The HTTP 501 Not Implemented
server error response status code means that the server does not support the functionality required to fulfill the request.
A response with this status may also include a Retry-After
header, telling the client that they can retry the request after the specified time has elapsed.
A 501
response is cacheable by default unless caching headers instruct otherwise.
501
is the appropriate response when the server does not recognize the request method and is incapable of supporting it for any resource.
Servers are required to support GET
and HEAD
, and therefore must not return 501
in response to requests with these methods.
If the server does recognize the method, but intentionally does not allow it, the appropriate response is 405 Method Not Allowed
.
If you have visited a web page and you are seeing 501
errors, these issues require investigation and fixing by server owners or administrators.
You can clear your browser cache for the domain, disable proxies if you are using one, or try again later to see if it works as expected.
A 501
response can occur if proxies cannot not handle request methods used in the context of HTTP Extension Framework (RFC 2774) applications.
This status can also occur in Web Distributed Authoring and Versioning (WebDAV) when a request method (SEARCH
, PROPFIND
) does not have a URL handler configured to process it.
Status
501 Not Implemented
Examples
Extension method not supported
In the following HTTP Extension Framework example, a client sends a request with a mandatory extension specified in the C-MAN
header.
The Connection
header specifies that these extensions are to be handled on a hop-by-hop basis.
A proxy refuses to forward the M-GET
method, and sends a 501
error in response:
M-GET /document HTTP/1.1
Host: example.com
C-Man: "https://www.example.org/"
Connection: C-Man
HTTP/1.1 501 Not Implemented
Specifications
Specification |
---|
HTTP Semantics # status.501 |
See also
- HTTP response status codes
510 Not Extended
- HTTP 501 errors in Microsoft ASP.NET documentation